“Supper” is similar to evening meal (cena). So, what is for supper? = ¿Qué hay de cena hoy? Sometimes mortals may “stay for supper” (se quedan a cenar).
There is also an expression: “to sing for one´s supper” (trabajárselo). Many moons ago, in 17th century Britain, minstrels (juglares), who performed in taverns, were paid with a meal. So the expression means to do something to get something in return.
So we all have to sing for our suppers!